I am by no means a FCP expert, however what I do is have a folder of standard template files which I created, and drag and "Insert" some items in the timeline (Warning, Title, Credits, etc) and I specify the clip length (3-5 secs), whereas with my URL watermark, I set the in point from where I want the watermark to start appearing on the video, and then drag the transparent .png watermark file to the "Superimpose" option in the canvas, and this automatically positions a watermark track above the video track in the timeline.
The default is usually 10 seconds I believe, but you can grab onto the end of the watermark track in the timeline, and drag it to the end of the clip, in order to have the URL display for the duration of the video.
If you Google "Final Cut Pro superimpose to add URL watermark" you can get links to lots of video and text tutorials which may be of further help since they offer various solutions to your issue.
Good luck...
ADG